[24.07] Intergalactic Extinction of High Energy Gamma-Rays

F.W. Stecker (NASA/GSFC)

We discuss the determination of the intergalactic
pair-production absorption coefficient derived by Stecker &
De Jager (1998) making use of an empirically based
calculation of the IR background spectrum given by Malkan &
Stecker (1998) which agrees well with recent data and
constraints on the IR background. We then show that present
air Cherenkov telescope observations of the energy spectra
of Mrk 421 and Mrk 501 are consistent with the amount of
absorption predicted by Stecker & De Jager and that new
HEGRA observations of the flaring spectrum of Mrk 501,
presented at the recent Harvard workshop, actually appear to
show just the amount of intergalactic absorption which we
have predicted. As a further test of our results, we give a
predicted spectrum, with absorption included, for
PKS2155-304. This XBL lies at a redshift of 0.12, making it
the highest redshift source yet observed at energies above
0.3 TeV. This source should have its spectrum steepened by
approximately 1 in its spectral index in the energy range
between 0.3 and 3 TeV and should show a marked absorption
turnover above about 6 TeV. We also discuss the
determination of absorption cutoffs for higher redshift
sources (Salamon & Stecker 1998) and their implications for
GLAST.

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